Ash management in circulating fluidized bed combustors

Abstract

Ash management in fluidized bed combustion systems means keeping the particle size distribution of the bed inventory in a given range. A dynamic particle population balancing model was developed for this purpose. It was successfully applied to a refuse-derived fuel fired combustor and a coal-fired circulating fluidized bed combustor. Both were large-scale commercial units. The model uses the concept of the attrited ash particle size distribution which represents the particle size distribution of the attrited ash including the generated fines and replaces the consideration of the particle attrition in the model calculations. The model offers the possibility to gain additional information about the particle size distributions and the solids mass flows at any location of the fluidized bed system. In addition, the model provides information about the dynamic behavior of the plant and about mean residence times of particle size classes in the plant. Uncertainties about the ash formation characteristics of fuels make the management of the bed inventory a very important issue. In this context the population balancing model is used to predict the plant behavior under various operating conditions. The results of the calculations carried out give useful information about the possibilities to manage the ash inventory of such a plant. It could be shown that the recirculation of a fine fraction of the bottom drain solids is a very effective method to manage the particle size distribution of the bed inventory. The calculation results further reveal that the mean residence time of particles is strongly dependent on their size.